The present invention relates to an improved mold core, and more particularly to a mold core having improved ejector pins for ejecting a molded piece held against the core.
In molding operations, it is often necessary to provide mechanical means to release the part from the mold when the mold opens. This is particularly true in blow molding of double wall parts, since the xe2x80x9ccorexe2x80x9d half of case molds generally contains projecting or xe2x80x9cmalexe2x80x9d sections over which the molten plastic parison drapes as the mold is closing. When the mold is closed, the molten material is pressed against the projecting mold sections by fluid pressure, causing adherence thereto. Additionally, the molten material is cooled, causing it to shrink and solidify and thereby adhere even more firmly to the projecting mold sections. Consequently, when the mold opens, the finished part is held tightly against the core half of the mold, and a substantial force is required to pull, pry or push the part off the mold. The preferred method of doing so is to use core mounted ejector pins movable perpendicular to the plane of the mold parting line to eject the part from the mold.
A problem often arises with the use of these pins, however, in that circumstances can exist in which the part adheres to the cavity side of the mold rather than the core. For example, parts may adhere to the cavity if the cavity contains pronounced textures whose interstices hold the molten plastic as it cools. In addition, cavities may contain sharp edges or undercuts in the area of molded feet, both of which may restrain the part in the cavity half of the mold. Moreover, cavities may contain inwardly projecting areas along their side walls or end walls, such as latch or handle mounts, around which the plastic drapes and adheres just as it does on cores. If the sum of these holding forces in the cavity exceeds the holding forces of the core, the part will stick in the cavity instead of on the core. In such a situation, the core ejector pins are obviously useless. Moreover, cavity ejector pins are impractical from the standpoints of machine and mold construction and appearance of the finished part. Therefore, removal of the part from the cavity half of the mold is difficult as well as disruptive to the regular cycling of the molding machine.
A known prior art method for overcoming this problem has been to cut semicircular or V-shaped groves in the vertical sidewalls of the core parallel to the mold parting line. Such a groove, known as a xe2x80x9ccore pull,xe2x80x9d is shown in FIG. 10. These grooves form undercuts which can greatly increase adherence of the part to the core, and which ensure that the part sticks thereon.
Reference is now made to FIGS. 10 and 11, in which prior art cores 110 are shown. Cores 110 include ejector pins 112 and core pull undercuts 114. Ejector pins 112 may have domed heads 116 (as shown in FIG. 10) or flush-mounted heads 118 (as shown in FIG. 11), and may include bushings 120 which are recessed within, and flush with, cores 110 and which have cylindrical outer surfaces.
The core pull undercuts 114, besides being unattractive, themselves cause problems. The ejector pins 112 have to exert more force in order to overcome the resistance of undercuts 114 than they would if undercuts 114 were not provided. This force, which is focused at locations remote from undercuts 114, tends to bow and deform the plastic covering flat surfaces of cores 110. Also, undercuts 114 must be made deep and/or long enough to pull the molded part out of a new cavity. However, a new cavity with new textures and new, sharp machined edges accentuates the problem of the molded part being stuck in the cavity. As the mold wears in, parts tend to stick less in cavities and it becomes desirable to reduce the depth or length of the core pull undercuts 114. However, there is then no easy way to make the large and unsightly core pull undercuts 114 any less so.
What is desired, therefore, is a mold core which ensures that the molded part adheres thereto when the mold is opened, which allows the molded part to be ejected off the mold, which produces a molded part which is aesthetically pleasing, which avoids bowing and deforming of the plastic covering flat surfaces of cores as the molded part is ejected off the mold, and which allows for easy variance of the force required to eject the molded part from the core as the cavity becomes worn in.
Accordingly, it is an object of the present invention to provide a mold core which ensures that the molded part adheres thereto when the mold is opened.
Another object of the present invention is to provide a mold core having the above characteristics and which allows the molded part to be ejected off the mold.
A further object of the present invention is to provide a mold core having the above characteristics and which produces a molded part which is aesthetically pleasing.
Still another object of the present invention is to provide a mold core having the above characteristics and which avoids bowing and deforming of the plastic covering flat surfaces of cores as the molded part is ejected off the mold.
Yet a further object of the present invention is to provide a mold core having the above characteristics and which allows for easy variance of the force required to eject the molded part from the core as the cavity becomes worn in.
These and other objects of the present invention are achieved by provision of a mold core having an upper surface and at least one hole passing therethrough. An ejector pin is disposed within at least one of the holes in the mold core. The ejector pin includes a bushing having an undercut portion protruding above the upper surface of the mold core for enhancing the adherence of a part molded on the mold core thereto. The bushing also has a hole passing laterally therethrough through which passes an ejector pin shaft. The ejector pin shaft, which includes a head, forces a part adhered to the mold core off of the mold core when so desired.
The protruding portion of the bushing is undercut in the form of a back-drafted angle, a circumferential rib, either indented or projecting, and of a partially round or V-shaped cross-section, a series of dimples, or of any other shape which will cause the molded plastic to adhere tightly to it. The head of the ejector pin may be located out beyond the bushing and may be dome-shaped, conical, or any of other various shapes. Alternately, the head of the ejector pin may be flush with the bushing or may be recessed within the bushing.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.